Standard x-ray images are projection images, in which a three-dimensional (3D) object is x-rayed and projected onto a two-dimensional (2D) surface. Depth information is thereby lost. It is however possible to reconstruct a 3D image data set of the object from a large number of projection images, which were recorded from different projection directions over an angle range of at least 180°. If the required angle range has been scanned completely, these so-called tomographic 3D reconstruction methods require no previous knowledge of the object to be reconstructed.
Specialist computed tomographs (CTs) specifically for the purpose are generally used for 3D imaging using X-ray tomography. These devices have an annular system, comprising x-ray detectors and an x-ray tube, that rotates about the patient at high speed, thereby recording numerous projection images from any size of angle range.
For image-controlled diagnostic or surgical interventions, in which 2D projection images are recorded in an ongoing fashion during the intervention, what are known as interventional x-ray systems such as angiography systems or (mobile) x-ray C-arm systems are used. In the latter case the x-ray tube and x-ray detector are attached to opposing arms of a C-arm that can be moved freely about the patient, to allow x-ray images to be recorded from any projection direction.
Such systems can currently be used to generate 3D image data sets in the same manner as with computed tomographs. To this end the x-ray tube and detector are rotated over an angle range of typically approx. 200° during recording and 3D reconstructions are then calculated. When vessels are recorded, this method is referred to as 3D rotational angiography.
Because of the relatively long rotation times of typically 3 to 20 seconds compared with CT, such imaging is however restricted to non-moving organs and structures. In the case of the heart and coronary arteries, tomographic methods cannot be used with interventional x-ray devices, as the heart motion during recording causes the structures of the heart to appear smudged in the reconstructed 3D image data set.
There are different approaches to creating 3D reconstructions of the heart from recordings by interventional x-ray devices such as angiography systems despite such problems.
With some methods for example the heartbeat is measured during image acquisition by recording an electrocardiogram (ECG). ECG gating is used so that only the projection images recorded during a relatively low-motion phase of the cardiac cycle are used for the reconstruction. What are known as “symbolic reconstruction methods” can be used to reconstruct specific, geometrically describable objects from these few projection images. Such objects are in particular blood vessels, which can be identified in the images by means of an intra-arterially injected high-contrast contrast agent. Such symbolic reconstruction methods, as disclosed for example in WO 97/49065 A1 and WO 01/85030 A1, have the disadvantage however that they make assumptions about the content of the image data set to be reconstructed and are also restricted to high-contrast objects. User interaction, e.g. the marking of specific landmarks in two or more images, is also essential.
Tomographic methods are also known, which use motion correction methods in order to be able to use more than just individual projection images for the reconstruction (see also C. Blondel, G. Malandain, R. Vaillant, N. Ayache: “4D deformation field of coronary arteries from monoplane rotational X-ray angiography”, Computer Assisted Radiology and Surgery, 2003 Proceedings, Vol. 1256 of ICS, London, UK, June 2003, Elsevier Science B.V.). The basic idea is to use motion correction methods during image reconstruction. Blondel's method for example also uses symbolic reconstruction methods to create a model of the coronary arteries. This model is then tailored to the overall sequence of the projection images, which change during the cardiac cycle, in order to generate a four-dimensional “deformation field”, showing the motion of the arteries during the course of the cardiac cycle. However this method also has the disadvantage of user interaction and is similarly restricted to high-contrast objects.